(1) Field of the Invention
The present invention relates to a video transmitting apparatus utilizing a frequency division multiplexing technique and a time division multiplexing technique, used in a on-demand video system, a surveillance camera system, or the like, which can transmit a plural sorts of video which cannot be transmitted by one transmission medium by using a plurality of transmission media, and can switch video over a plurality of transmission media with less distortion of the video.
(2) Related Art
In a field of cable television system, surveillance camera system or the like, which needs to transmit a plural pieces of video information, there is employed a video transmitting apparatus which multiplexes video information on a transmission medium, in these years.
Hereinafter, a known video transmitting apparatus will be described. Japanese Patent Application Laid-Open Publication No. 5-7330 discloses a transmitting apparatus using a transmission medium on which a plural pieces of video information can be multiplexed. The video transmitting apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 5-7330 is configured as a monitor camera system. However, it is possible to modify such the monitor camera system into a video transmitting apparatus in a cable television system, by assuming that video inputting units (cameras) are not dispersed but concentrated in a head-end portion and by employing video players as the video inputting units instead of TV cameras.
FIG. 16 shows a structure of the known transmitting apparatus. In FIG. 16, reference numerals 1601-1 through 1601-n denote video inputting units (cameras), 1602 a transmission medium on which video information inputting from the video inputting units is multiplexed and transmitted, 1603 a video outputting unit (a monitor) for displaying the transmitted video information, 1604-1 through 1604-n video information fixedly transmitting apparatus each for fixedly outputting the video information obtained by the corresponding video inputting unit 1601-1, 1601-2, . . . or 1601-n on a specific channel to the transmission medium 1602, and 1605 a video information receiving unit for selecting video information on a specific channel among a plurality of channels multiplexed on the transmission medium 1602 and outputting the same to the video displaying unit 1603.
Next, description will be made of an operation of the video transmitting apparatus with the above structure. The video information fixedly transmitting apparatus 1604-1 through 1604-n modulate video information obtained by the video inputting units (cameras) on different fixed channel resources, and multiplex the modulated video information on the transmission medium 1602. On the other hand, the video receiving apparatus on the receiving side demodulates video information signals among the multiplexed video information using a designated channel resource, and the video outputting unit (a monitor) 1603 displays the demodulated video information.
The operator can display video obtained by an arbitrary video inputting unit (camera) by changing a demodulation channel of a video information receiving unit.
However, the above video transmitting apparatus has the following disadvantages.
Since channel resources on the transmission medium are limited, the number of sorts of video that is required on the video display's side (monitor's side) at the same time can not be larger than the number of channel resources.
If two or more systems of the transmission medium are provided, it is impossible to display video obtained by a video inputting unit (camera) connected to a certain system on the video outputting unit (monitor) connected to another system.
In the case where the video (camera) is switched by changing the demodulation channel on the video display's side, a screen is distorted when the channel is switched in the middle of a video frame. This phenomenon is generated not only in a switching video connected to the same transmission medium, but also in a switching video over a plurality of transmission media.
Further, an apparatus (a video display requesting unit 1606) used to request a video display by the operator is required to be placed close to the video outputting unit (monitor).
In the light of the above disadvantages, an object of the present invention is to simultaneously supply plural sorts of video larger in number than channel resources possessed by one transmission medium by using a plurality of transmission media, enable switching of video over a plurality of transmission media, enable video transmission without distortion of video at the time of a switching of the video by synchronizing vertical synchronization signals in the transmission media, and enable video switch request from a place away from a monitor.